Protoplasts were produced from A. nidulans mycelium using Trichoderma lytic enzyme. The influence of KCl and MgSO4 as stabilizer systems on the morphological variation of protoplasts produced during digestion and the pattern of release from hyphae were compared. Apparently protoplast release in the presence of KCl followed a sequential fractionation of the hyphae with early protoplasts originating from the tip regions and late protoplasts from the distal regions. In MgSO4-stabilized systems the hyphae were disrupted in a less ordered fashion. Between 12 and 16% of the mycelial protein was recovered in protoplast form using the systems described. The level of chitin synthase (EC 2.4.1.16) and the capacity for trypsin-activation of the enzyme in protoplast fractions was investigated. In KCl-stabilized systems, early (1 h) fractions possessed higher specific activities than later fractions. Activatable enzyme was low in the early fraction but was present at high levels in later fractions. These observations are consistent with a model relating active and activatable enzyme to hyphal growth.